A flat panel display possesses advantages of being ultra thin, power saved and radiation free and has been widely utilized. Present flat panel displays mainly comprise LCDs (Liquid Crystal Display) or OLEDs (Organic Light Emitting Display). Regarding of flat panel displays, a Thin Film Transistor Liquid Crystal Display (TFT-LCD) has characteristics of small volume, low power consumption, relatively lower manufacture cost, radiation free and etc. which has dominated the main flat panel display market.
The main principle of the liquid crystal display screens is that the back light creates white light, and after the light passes through the polaroid of the array substrate, the polarization state of light is changed because of the different twisted angles of the liquid crystal. After the light passes through the polaroid of CF, the different brightnesses are resulted for the different pixels. In the full color display, a color filter layer is added on the CF substrate to obtain different mixed light intensities for different pixels RGB of the three primary colors to realize the color display.
For increasing the aperture ratio and reducing the influence of the pixel parasite capacitance, the present COA (Color Filter on Array) technology has been widely utilized. The COA technology is a kind of integration skill of combining the color filter and array substrate as one. That is, the color resist is coated on an accomplished array to form a color filter layer, which can improve the issue of low aperture ratio of the traditional color filter.
In a traditional structure of a liquid crystal display, two glass substrates sandwich the liquid crystal layer inbetween. The thin film transistors (TFT) are manufacture on one glass substrate for driving the orientations of the liquid crystal for controlling the display of every pixel; the color filter is formed on the other glass substrate to be utilized for forming colors of every pixel. The Color Filter on Array integration technology is to manufacture the TFT and color filter on the same glass substrate. Such technology possesses advantages below:
(1) Self-alignment of upper and lower substrates. In the traditional structure, the two glass substrate require the alignments of the corresponding pixels on the two glass substrates and a cell process is implemented, i.e. the alignment procedure after the manufactures of TFT and color filter are manufactured on the two glass substrates in the front and the rear. The accuracy of the cell process directly influences the quality of the liquid crystal display. Once the COA technology is utilized, because the TFT and color filter are both provided on the same glass substrate and self-alignment is inherently achieved. There is no need for an additional alignment procedure. As a result, the process can be simplified and the product quality is improved.
(2) Manufacture cost reduction. The color filter takes a large ratio in the manufacture cost of the liquid crystal displays. The manufacture cost can be effectively reduced by forming the color filter on the TFT and eliminating the processes of manufacturing the color filter; besides, the color filter significantly affects the performance but the makers of the color filter and the liquid crystal display are different. The man power, the money can be wasted for the considered cooperation and the quality is hard for guarantee. On the contrary, the COA technology can solve the aforesaid problems and reduce the manufacture cost.
(3) Improvement of production performance. The aperture ratio is increased and the penetration ratio is raised. Once the Color Filter on Array is implemented, the line width of the black matrix can be narrowed down from 27 μm to 10 μm. The aperture ratio can be increased with 15%. The corresponding penetration ratio is enormously raised, too
However, the thickness of the Color Filter on Array is thicker and in general has a thickness of several micrometers. The vias for connecting the signal lines and the pixel electrodes have to be large for guaranteeing the effective contacts. Such larger vias may cause issues of bubbles, decrease of the aperture ratio, the orientation disorder of the liquid crystal and abnormal alignment and etc.